the differences between RANK, DENSE_RANK and ROW_NUMBER are how they handle dups.

Rank- Unique Rank for each row, dupes skip the next rank. Eg: 1, 2, 3, 3, 5. Rank 4 is skipped.

Dense Rank- Unique Rank for each row but Dupes aren't skipped. Eg, 1, 2, 3, 3, 4, 5. Rank 4 isn't skipped, it comes after the dupes.

ROW_NUMBER- Unique rank for each row regardless of dupes. Every row gets ranked in order, duplicates are ignored. Eg, 1, 2, 3, 4, 5, 6.

Did you change the order by statement? Got a similar error and it was resolved after i cahnged the order by to average_salary.

Hi, Amna. CONCAT_WS() is just like CONCAT() but you get to set a separator beforehand -- lemme explain. If I want to print out:
'I-like-cats' I can do it as follows:
SELECT
CONCAT_WS ('-', 'I', 'like', 'cats')
FROM users;